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The easiest way is to hold down the Alt key and type "156", then release the Alt key. You will see a British pound (£) sign appear. However, sometimes "pound" means the number sign (#), but that's mostly used in voicemail prompts.
If you use the first pound symbol a lot, you can either keep it on the clipboard, or consider remapping a key you never use (like the tilde, or ~) to type that character (see http://www.annoyances.org/exec/show/article02-022).